Overactive Thyroid Gland (Hyperthyroidism, Thyrotoxicosis)

The thyroid gland is a double-lobed gland that lies just below and in front of the voice box. It resembles a fatty mass and can extend to the upper part of the chest cavity. An abnormally enlarged thyroid gland is known as a goiter and is often an indication of a diseased state. This rather unassuming gland has several important functions, of which controlling the body’s metabolic rate is its primary function.

There are two thyroid hormones secreted by the thyroid gland – thyroxine (T4) and triiodothyronine (T3). These hormones raise the body’s metabolism by increasing the number of mitochondria (‘power houses’) in the cell and also increases glucose utilization. The thyroid hormones also affect protein synthesis, breakdown of fats from fatty tissue, and absorption of nutrients from the gut. Clinically, the effects of thyroid hormones is seen as an increase in heart rate, breathing rate, cardiac output and weight loss.

The thyroid gland’s activity is regulated by a direct feedback mechanism. When the body requires more thyroid hormones, the hypothalamus releases thyrotropin-releasing hormone (TRH) which then acts on the pituitary gland. In response, the anterior pituitary secretes thyroid-stimulating hormone (TSH) which then travels by way of the bloodstream to the thyroid gland. Here it stimulates the thyroid gland to increase the production and secretion of the thyroid hormones.

What is an overactive thyroid gland?

An overactive thyroid gland is where there is hyperfunctioning (“over-functioning”) of the thyroid gland known as hyperthyroidism leading to raised levels of thyroid hormones (thyrotoxicosis). Overactivity of the thyroid gland may stem from pathology within the thyroid gland itself (primary hyperthyroidism). Alternatively it may arise from overactivity of the pituitary gland where there is an increase in thyroid-stimulating hormone (TSH) which may or may not be related to hypothalmic influence (secondary hyperthyroidism).

Hyperthyroidism vs Thyrotoxicosis

The terms hyperthyroidism and thyrotoxicosis are often used interchangeably and this may be confusing. In hyperthyroidism, the thyroid gland is hyperfunctioning and since its main function is the production and secretion of thyroid hormones, there will be clinical features of elevated thyroid hormones known as thyrotoxicosis. It is, however, possible for thyrotoxicosis to arise without hyperthyroidism, for example with exogenous (external) thyroid hormone administration. Hyperthyroidism is the most common, but not the only, cause of thyrotoxicosis.

Of these, Graves disease is by far the most common accounting for over 70% of hyperthyroidism cases.

Medication and Substances

Iodine-induced thyrotoxicosis (drug-induced thyrotoxicosis) is a result of excess iodine which increases thyroid hormone production and therefore secretion. This may be seen with the use of :

certain drugs high in iodine like amioidarone

iodine supplementation

radiocontrast iodine for radiological investigation

Factitious thyrotoxicosis (factitia thyrotoxicosis) is a result of excessive intake of thyroid hormones. This may be seen with accidental or intentional overuse of thyroid hormone drugs by patients with hypothyroidism (underactive thyroid gland). The use of these drugs by people who have normal thyroid functioning may also lead to thyrotoxicosis and this is seen when it is misused as a weight loss drug.

Babies

Neonatal thyrotoxicosis may occur in newborns when the mother has Graves disease. Nonautoimmune autosomal dominant hyperthyroidism is seen in infants where there is continuous stimulation of the thyroid gland due a mutation of the TSH receptor gene.

Growths and Cancer

Pituitary adenomas may increase TSH production and secretion leading to overstimulation of the thyroid gland. These are known as TSH-secreting pituitary adenomas.

Choriocarcinoma or a hydatidiform mole may lead to excess hCG which can then stimulate the thyroid gland since hCG is normally a weak stimulator. Women with hyperemesis gravidarum may also experience thyrotoxicosis.

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